1. Field of the Invention
The present invention relates to a control device for an internal combustion engine which carries out ignition timing control by detecting knocking of an internal combustion engine and can enhance knocking detection precision.
2. Description of the Related Art
An internal combustion engine for a vehicle is equipped with a knocking control device for controlling an ignition timing by detecting knocking, thereby enhancing fuel consumption, performance, etc. A generally used knocking control device comprises a band pass filter for extracting a specific knocking frequency from vibration frequencies detected by a knocking sensor, a high pass filter for extracting only AC components from the output of the band pass filter, a negative voltage clipping bias circuit for extracting positive half waves by superposing a bias voltage on the output of the high pass filter, an amplifying circuit for amplifying the positive half waves thus biased, a peak hold circuit for holding the peak value of the half wave voltage thus amplified for a predetermined time, and it is used to preventing malfunction and breakdown of an operation amplifier used in the amplifying circuit.
For example, there will be described the operation and characteristics of the generally used knocking control device thus constructed when the internal combustion engine is rotated at a low velocity. The band pass filter extracts a specific frequency component from the output of the knocking sensor and outputs an AC voltage proportional to a knocking signal. When the peak value of the voltage thus output is represented by Vm and a biasing DC output voltage generated by the negative voltage clipping bias circuit is represented by Ve, a voltage held by the peak hold circuit is equal to Vm+Ve, and thus the knocking control is carried out on the basis of this voltage. However, the actual knocking signal is equal to Vm, and thus the bias voltage Ve set for the purpose of protecting the amplifying circuit serves as an error to the knocking signal.
When the internal combustion engine is rotated at a high speed, the amplitude of the knocking signal is large. However, with respect to the time constant which is formed on the circuit construction, the difference in time constant between the charging operation and the discharging operation appears remarkable in a high pass filter comprising a CR circuit of a capacitor and a resistor. The unbalance of the time constant as described above occurs because a reverse blocking diode provided to the negative voltage clipping bias circuit varies the circuit constants of the charging circuit and the discharging circuit to the capacitor of the capacitor. This variation acts so that the charging time constant is larger than the discharging time constant, and the capacitance terminal voltage of the high pass filter is shifted in parallel toward the positive voltage side with respect to the earth voltage. As a result, a peak voltage Vma higher than the actual knocking signal Vm is output, and further this voltage is added with the DC output voltage Ve of the negative voltage clipping bias circuit. Therefore, the voltage held by the peak hold circuit is equal to Vma+Ve, and thus the error is enlarged. This error is increased as the knocking signal is large, in other words, as the rotational number is increased.
In the conventional generally-used knocking control device, the error based on the DC output voltage Ve of the negative voltage clipping bias circuit and the error based on the unbalance of the charging and discharging time constants in the high pass filter are added with each other, and thus a normal peak hold voltage to the knocking signal cannot be achieved, so that the ignition time control cannot be performed with high precision. Furthermore, the bias voltage Ve and the error based on the unbalance of the charging and discharging time constants shift the peak hold voltage to the high voltage side. Therefore, as described later, the knocking signal voltage reaches a predetermined voltage determined by a power source voltage under a relatively low rotation speed, and the knocking signal is clipped to the predetermined voltage, so that the dynamic range is narrowed.